Firstly I think most of the people here are in the Republic, so it might be hard to get detailed advice, but from a quick look it seems like it works similarly in the North to here.
Generally there will be a property management company that holds the common areas and is responsible for the upkeep etc., and each unit in the development has a "share" in this company - so you as the owners own that company. Because you all don't want to be worrying about running that company, you'll tend to appoint a few Directors (probably some of your neighbours) to run it for a year and they will appoint a management agent to look after the property, collecting fees etc. (Charterhouse in your case). But as I say that agent is appointed by the management company that you own and is collecting the fees for a company that you own.
As you own the management company you should be turning up to its regular meetings and taking an active part in voting on what will be done, if you have a problem with the quality of work the agent is doing you just shout that out and if others agree you either warn them or replace them with a new company, just like you would change to a new car insurance provider if you were not happy.
FWIW as one of those neighbours acting as a Director on an owners management company, it's incredibly frustrating to have other owners moaning about fees and hassling you about getting a light bulb fixed when you're running their company for them, so I'd ask you to go into it with an open mind initially
. If you find it's a complete mess and the developer still holds the management company or the Board, then that's different and you'd probably want to get other members together and try to rectify it (usually requires legal advice).
As cremeegg pointed out, in the Republic at least you are not able to sell a property if there are management company fees outstanding, the buyers solicitor will pick it up in due diligence and insist they are paid by the vendor. In the company I'm involved with we add interest to reduce late payments.
Again you'll need to confirm exactly how it works in NI, but if it is how it should be here then you are likely an owner of the management company and need to figure out when the meetings are so you can start to have more of a say at them.